Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry having a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, physical vapor deposition, plasma enhanced chemical vapor deposition, plating, planarizing, etching, etc.). The dies typically include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are the external electrical contacts on the die through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. The wafer is then thinned by backgrinding and the dies are separated from one another (i.e., singulated) by dicing the wafer. After the dies have been singulated, they are typically “packaged” to couple the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines, and ground lines.
Conventional processes for packaging dies include electrically coupling the bond-pads on the dies to an array of pins, ball-pads, or other types of electrical terminals, and then encapsulating the dies to protect them from environmental factors (e.g., moisture, particulates, static electricity, and physical impact). In one application, the bond-pads are electrically connected to contacts on an interposer substrate that has an array of ball-pads. For example, FIG. 1 schematically illustrates a conventional packaged microelectronic device 2 including a microelectronic die 10, an interposer substrate 20 attached to the die 10 with an adhesive 30, a plurality of wire-bonds 32 electrically coupling the die 10 to the substrate 20, a casing 50 protecting the die 10 from environmental factors, and a plurality of solder balls 60 attached to the substrate 20. After assembly, the adhesive 30 and the casing 50 are typically cured to form a robust packaged device 2.
Another type of microelectronic device is a “flip-chip” semiconductor device. These devices are referred to as “flip-chips” because they are typically manufactured on a wafer and have an active side with bond-pads that initially face upward. After manufacture is completed and a die is singulated, the die is inverted or “flipped” such that the active side bearing the bond-pads faces downward for attachment to an interposer substrate. The bond-pads are usually coupled to terminals, such as conductive “bumps,” that electrically and mechanically connect the die to the interposer substrate. The bumps on the flip-chip can be formed from solders, conductive polymers, or other materials. In applications using solder bumps, the solder bumps are reflowed to form a solder joint between the flip-chip component and the substrate, which leaves a small gap between the flip-chip and the interposer substrate. To enhance the integrity of the joint between the microelectronic component and the substrate, an underfill material may be introduced into the gap. The underfill material bears some of the stress placed on the components and protects the components from moisture, chemicals, and other contaminants. After flowing the underfill material into the gap between the flip-chip component and the substrate, the underfill material is cured.
Conventional methods for curing underfill materials, encapsulants, adhesives, and other compounds include either heating the curable material with various techniques or irradiating the curable material with microwave energy at a fixed frequency. One advantage of irradiating the material is that the time required to cure the material is reduced. Curing materials with microwave energy at a fixed frequency, however, has several drawbacks. For example, when microwave energy is applied to a microelectronic substrate, arcing and/or excessive heat accumulation may occur and cause localized damage to the substrate and the component to which the substrate is mounted. Arcing results from the build-up of a charge differential between different components or between one or more of the electronic elements within the components. When the difference in potential exceeds the resistance of a dielectric medium, such as air, the result is a release of the built-up charge through the dielectric medium manifested by an arc between the two oppositely charged components. Moreover, microwave energy may heat certain portions of the conductive circuitry more rapidly than other portions, which may damage the circuitry.
One existing approach to address such drawbacks of curing materials with fixed-frequency microwave energy is to vary the frequency of the applied microwave energy. Sweeping the frequency prevents the build-up of a charge differential and the excessive accumulation of heat. As a result, variable frequency microwave curing typically avoids arcing and the associated localized damage to microelectronic components. One problem with this approach, however, is that applying microwave energy over a range of frequencies may adversely affect other components within the microelectronic device. For example, doped silicon, polymeric random access memory, and chalcogenide are irreversibly changed when exposed to microwave energy at certain frequencies. Specifically, with regard to doped silicon, microwave energy can cause dopant atoms to diffuse throughout a substrate and render the doped structure and other features in the substrate defective. As a result, there exists a need to improve the process of curing materials in microelectronic devices.